Problem: In history class, the girl to boy ratio is $7$ to $3$. If there are a total of $30$ students, how many boys are there?
Answer: A ratio of $7$ girls to $3$ boys means that a set of $10$ students will have $7$ girls and $3$ boys. A class of $30$ students has $3$ sets of $10$ students. Because we know that there are $3$ boys in each set of $10$ students, the class must have $3$ groups of $3$ boys each. There is a total of $9$ boys in history class.